1. Field of the Invention
The present invention relates to an image processing apparatus that compression-encodes image data through conversion into frequency bands or an image processing apparatus that expansion-decodes image data having been compression-encoded through conversion into frequency bands.
2. Related Background Art
Recent digital video cameras and digital still cameras use image pickup elements having high sensitivities and are capable of setting a maximum gain of automatic gain control (AGC) to be extremely high at the time of digital conversion.
In such a system, even when the color temperature of an object deviates slightly from a preset color temperature of an image pickup element, there is a large fear in that lost of color balance may conspicuously appear depending on the magnitude of a gain set with reference to illuminance of the object. As a result, at the time of low illuminance, unnatural coloring occurs in an image due to a noise component. Conversely, at the time of high illuminance, there occurs a phenomenon in which a part of an image that is originally white has a tinge of blue or a tinge of red. In view of this problem, a conventional camera performs color suppression processing for a chrominance signal processing system, thereby avoiding an unnatural coloring phenomenon.
In a camera processing circuit of such a camera, generally, a non-linear chrominance signal processing apparatus performs color suppression processing on chrominance data in accordance with illuminance, and resultant chrominance data is corrected so as to be suited for display characteristics through gamma correction and the like. On the other hand, luminance data is subjected to correction corresponding to the color suppression processing performed on the chrominance data.
Now, a schematic construction of a conventional digital video camera will be described with reference to FIG. 15 (digital still camera also has the illustrated construction). Also, a schematic construction of a non-linear chrominance processing circuit 340 of the digital video camera will be described with reference to FIG. 16.
In FIG. 15, an electric signal obtained by picking up an image of an object with a lens and image pickup system 300 is inputted into a camera processing circuit 310. In the camera processing circuit 310, the electric signal is converted into a digital signal by an A/D conversion circuit 320 and the digital signal is divided into components that are luminance data (Y) and chrominance data (C) by a component conversion circuit 330. Following this, the luminance data and the chrominance data are processed in a non-linear color processing circuit 340 to be described in detail later, and are corrected in a correction circuit 350. Then, an output from the camera processing circuit 310 is compression-encoded by a compression encoding circuit 360 and is recorded on a recording medium 370.
Next, a construction of the non-linear chrominance processing circuit 340 will be described with reference to FIG. 16. When an image signal is inputted into the non-liner color processing circuit 340, first, a low-pass filter 341 extracts a low-frequency component from luminance data of the input image signal. Then, an illuminance detection unit 342 detects illuminance of an image from the extracted low-frequency component.
Here, the luminance data contains data showing each part (bright part, for instance) of an image, in which brightness greatly changes, and data showing the overall brightness of the image. The former data is contained more largely in a high-frequency component and the latter data is contained more largely in the low-frequency component. Accordingly, the illuminance of the image can be detected by extracting the low-frequency component of the luminance data and comparing the extracted component with a predetermined value.
Here, FIGS. 17A, 17B, and 17C each schematically show a frequency distribution of luminance data, with FIG. 17B relating to a normal image, FIG. 17A relating to a high-illuminance image, and FIG. 17C relating to a low-illuminance image. As can be seen from these drawings, generally, even if illuminance is lowered, data in a high-frequency region (region in which brightness and darkness greatly change) does not so greatly change as compared with data in a low-frequency region, which means that detection of illuminance using low-frequency data (a hatched region in the drawings) is effective.
Next, referring again to FIG. 16, an operation in the case of a low-illuminance image having the frequency distribution shown in FIG. 17C will be described. When the low-frequency component of the luminance data is smaller than the predetermined value, the illuminance detection unit 342 detects that the image is at low illuminance and controls a chrominance data gain correction unit 343 so as to reduce a gain, thereby reducing the influence of a chrominance component in a region, where the illuminance is equal to or lower than the predetermined value, as shown in FIG. 18. As a result of this processing, the chrominance data is limited to a specific color (usually to achromatic color). Then, a correction signal generation unit 344 generates a correction signal of the luminance data for realizing a color balance with which the specific color becomes the achromatic color, and outputs the correction signal.
Originally, a high gain is set by the AGC for the low-illuminance image and therefore the noise component is amplified. If the noise component is contained in the chrominance data, this results in a situation where there occurs unnatural color-unevenness that is not originally possessed by the object. By performing the non-linear processing in the manner described above, however, in the case of illuminance equal to or lower than the predetermined value, a chrominance level is uniformly limited (suppressed), so that such unnatural color-unevenness does not occur and an image only having brightness and darkness indicated by the luminance data is obtained. As a result, image data that is visually favorable is obtained.
Next, an operation in the case of a high-illuminance image having the frequency distribution shown in FIG. 17A will be described with reference to FIG. 16. When the low-frequency component of the luminance data is larger than the predetermined value, the illuminance detection unit 342 detects that the image is at high illuminance and controls the chrominance data gain correction unit 343 so as to reduce the gain, thereby reducing the influence of the chrominance component in a region, where the illuminance is equal to or higher than the predetermined value, as shown in FIG. 18. As a result of this processing, the chrominance data is limited to a specific color (usually to white). Then, the correction signal generation unit 344 generates a correction signal of the luminance data for realizing a color balance with which the specific color becomes white, and outputs the correction signal.
Originally, a low gain is set by the AGC for the high-illuminance image and then the level of the chrominance data is large in this case. Therefore, a slight error in white balance due to a change in color temperature is emphasized, which results in a situation where the image has a tinge of blue or a tinge of red and a strange color drift occurs. By performing the non-linear processing in the manner described above, however, in the case of illuminance equal to or higher than the predetermined value, the chrominance level is uniformly limited (suppressed), so that no unnatural coloring occurs and an image only having brightness and darkness indicated by the luminance data is obtained. As a result, image data that is visually favorable is obtained.
However, the color-unevenness at the time of low illuminance is a problem that occurs due to an increase of the high-frequency noise component with respect to the low-frequency signal level. In order to solve this problem, with the construction described above, fluctuations due to the high-frequency noise component are cancelled out by uniformly limiting signal levels in all frequency bands. This leads to a situation where an object that originally is colored even under low illuminance is also subjected to the image correction to the achromatic color.
Also, there is a problem that the picked-up color at the time of high illuminance appears so that a slight color drift exerts an excessive influence since the absolute value of the low-frequency signal level is large. In order to solve this problem, with the construction described above, the low-frequency signal level is suppressed by uniformly limiting the signal levels in all frequency bands. This leads to a situation where an object that originally includes color changing even under high illuminance is also subjected to the image correction to white.
That is, with the conventional method, information originally possessed by an object is unnecessarily deleted as a result of visual effect processing.